The present invention is related to the field of cellular telecommunications. More particularly, the present invention involves a method and apparatus for improving the downlink Carrier-to-Interference ratio (C/I) by means of antenna pattern downtilt while maintaining uplink signal strength.
In a cellular telecommunications system (e.g., a cellular mobile telecommunications system) maintaining and/or improving speech quality is of great importance. One factor which can significantly and adversely affect speech quality is the presence of co-channel interference. Co-channel interference occurs when two or more cells (i.e., co-channel cells) located adjacent to one another or in relatively close proximity to one another reuse (i.e., share) the same frequency or set of frequencies. In essence, a signal being transmitted over a reused frequency in one cell is perceived as interference in the other cell.
One way in which co-channel interference can be avoided is to assign a group of dedicated frequencies to each cell in the system so that no two cells reuse the same frequency. Unfortunately, there are but a limited number of frequencies available to cover an ever increasing demand for cellular service. Accordingly, assigning a dedicated group of frequencies to each cell is generally not a feasible solution to the co-channel interference problem.
Another technique which is often used to minimize, rather than eliminate, co-channel interference involves maximizing reuse distance. Reuse distance is generally understood to be the distance between two cells (i.e., co-channel cells) that reuse the same frequency or set of frequencies. As one skilled in the art will readily understand, as reuse distance increases, co-channel interference decreases (i.e., signal strength diminishes with distance). However, a higher reuse distance inherently implies a less efficient utilization of the available frequency spectrum. Despite increased demand for cellular service, the available frequency spectrum has remained constant. In response, cellular service providers are forced to increase the capacity in their systems and use the available frequency spectrum in the most efficient manner. This in turn tends to decrease rather than increase reuse distance. Consequently, techniques that rely on increasing reuse distance to counter the effect of co-channel interference are not an overly practial or attractive solution in terms of efficient spectrum usage.
Yet another method for reducing co-channel interference involves adjusting the orientation or tilt angle of the base station antenna. In general, the base station antenna transmits and receives telecommunications signals to and from the various mobile units operating within the corresponding cell, herein referred to as the target cell. By redirecting the antenna so that the antenna beam points further and further below the horizon, the energy associated with the antenna beam is, to a greater extent, directed into the target cell and away from any adjacent cells or co-channel cells in close proximity to the target cell. The objective of directing the main beam of the vertical antenna pattern towards a point below the horizon is to reduce the antenna gain towards interfering cells without causing a large reduction of the signal strength in the target cell.
Consequently, uplink interference received by the base station antenna in the target cell is reduced, since the antenna gain to interfering mobile stations in co-channel cells is reduced. Also, downlink co-channel interference received by the mobile units operating in co-channel cells caused by transmissions emanating from the base station antenna in the target cell is reduced due to a lower antenna gain. Tilting all antenna patterns to a point below the horizon to reduce co-channel interference, therefore, leads to a gain in the Carrier-to-Interference ratio (C/I) in both uplink and downlink in the system. This gain can be utilized to improve the quality of the communication or to allow a tighter reuse pattern.
Tilting of the vertical antenna beam can be achieved either mechanically or electrically. A particular mounting arrangement can be used to achieve the desired tilt by mechanical means. In a linear array antenna, the radiating elements have a phase excitation resulting in an electrically tilted main beam.
As with the other above-identified techniques for avoiding or minimizing co-channel interference, redirecting the base station antenna to reduce co-channel interference is not without trade-offs. The increase in C/I in the interfered cell may result in an undesired reduction of coverage at the border of the interfering cell. This is best illustrated with reference to FIG. 1. By increasing the tilt angle 101 of FIG. 1, the peak of the antenna beam 103 is directed inward; away from the target cell's boundary 105. The signal strength or Carrier-to-Interference ratio (C/I) will undesirably decrease for those signals being transmitted between the base station and mobile units operating in the target cell at or near the target cell boundary 105. Stated differently, an increase in the tilt angle 101 of the antenna beam 103 effectively reduces the coverage area of the target cell despite the fact that it reduces the level of uplink co-channel interference in the target cell and downlink co-channel interference in nearby co-channel cells.
The maximal coverage reduction thus limits the C/I gains obtainable by antenna tilting. One approach to this tradeoff is addressed in a co-pending U.S. patent application Ser. No. 08/941,204 entitled "Method and Apparatus for Optimizing Antenna Tilt" where a method for applying antenna downtilt is proposed. The optimum downtilt angle for each base station is derived from system measurements of the uplink interference in the target base station and downlink coverage measurements in the mobile station.
In order to increase capacity or improve quality in present systems, a significant reduction in downlink interference is required. To get a sufficient gain in C/I without degrading coverage too much, tall antennas with narrow vertical beams will have to be employed. Also, the ratio of antenna mounting height with respect to surrounding objects, compared to the site to site distance will have to be kept at a higher level. In many systems, capacity is limited by downlink C/I. Therefore, there exists a need for a system that provides a substantial gain in downlink C/I while maintaining the coverage limiting signal strength in the uplink and without the requirement of having very tall antennas.